1. Field of the Invention
The present invention relates to sample and hold circuits, and in particular, to voltage sample and hold circuits for low leakage charge pump circuits in phase lock loop applications.
2. Description of the Related Art
Referring to FIG. 1, charge pump circuits are often used in phase lock loop (PLL) applications for driving the voltage controlled oscillator (VCO) with a voltage filtered by a low pass loop filter Zf. Pump-up PU and pump-down PD signals, which are active low and high, drive a P-type metal oxide semiconductor field effect transistor (P-MOSFET) P1 and an N-MOSFET, respectively. These signals PU, PD are active during the active state of a control signal .phi.1. During the inactive state of this control signal .phi.1, the charge pump drive signals PU, PD are disabled by opening switches S1U and S1D, and the charge pump transistors P1, N1 are turned off by closing switches S2U and S2D. This places the output of the charge pump circuit, i.e., at the mutually connected drain terminals of the output transistors P1, N1, in a high impedance state. This disablement of the charge pump circuit is often done as a way to open the loop of the PLL to allow direct modulation of the VCO with some form of modulation signal MOD.
A problem with this, however, is the effect of leakage currents through the transistors P1, N1 upon the voltage provided to the tuning input of the VCO. This voltage will gradually drift due to the charging or discharging of this node by way of the net leakage current Ioff which is the difference between the leakage current Ileakn of transistor N1 and the leakage current Ileakp of transistor P1 (Ioff=Ileakn-Ileakp). This is due to the typical mismatch between leakage currents for N-MOSFETs and P-MOSFETs and becomes worse as one transistor allows more leakage current than the other.
Typically, the dominant component of these leakage currents is the subthreshold currents of the transistors P1, N1. As MOSFETs are scaled down to smaller device geometries, subthreshold current becomes more substantial as compared to the normal "on" current.
One conventional technique for seeking to isolate the tuning input of the VCO is to introduce an in-line switch S3 which is closed during application of the pump-up PU and pump-down PD signals and opened during the off, or high impedance, state of the charge pump circuit. However, one significant problem with this technique is that switch S3, when it is turned off, does not prevent the output node of the charge pump from drifting due to the net leakage current, Ioff. This results in a voltage difference across switch S3, which will cause the frequency of the output of the VCO to drift.
Referring to FIG. 2, one conventional technique for seeking to prevent unequal voltages from appearing across switch S3 is to use a feedback amplifier A1 in the form of a voltage follower for equalizing the voltages at the tuning input of the VCO and the output node of the charge pump circuit. When switch S3 is opened, switches S4A and S4B are closed, thereby closing this feedback loop and equalizing the voltages on both sides of switch S3. (This is a technique taught by U.S. Pat. No. 4,544,854, issued Oct. 1, 1985, and entitled "Analog Switch Structure Having Low Leakage Current," the disclosure of which is incorporated herein by reference.) However, this technique is not without its own problems. During the period that the voltages on either side of switch S3 are being equalized by the feedback action of amplifier A1, the noise present at the input of the amplifier A1 is now introduced to the tuning input of the VCO, thereby introducing yet another form of noise into the VCO output.
Accordingly, it would be desirable to have a technique for maintaining the voltage at the output node of a charge pump circuit, notwithstanding leakage currents affecting such node, without introducing additional noise.